1. Field of the Invention
The instant invention relates to a method and apparatus for reducing the error of a direction finding system. More specifically, the invention relates to a method and apparatus for reducing the amplitude modulation error in a spinning antenna direction finding system. In particular, the invention relates to a method and apparatus, in two embodiments, for using two distinct antennas to reduce the amplitude modulation error of the spinning antenna in a direction finding system.
2. Description of Prior Art
Most direction finding systems customarily incorporate a direction-finding receiver mechanically and/or electronically tunable for receiving various frequencies broadcast by radiant energy sources. Usually, such a system also includes an azimuthally rotatable directional antenna. Both the Navy and Air Force have extensive use of rotating direction-finding (DF) antennas in airborne and surface applications. In electronic-countermeasures (ECM) applications, it is also desirable to include automatic gain control (AGC) circuitry to prevent overload of receiver while maintaining sensitivity to very high and very low received signals. It is common knowledge that in spinning antenna DF systems, considerable bearing error of the received radiant energy source can occur due to amplitude modulation of the source signal. The instant invention discloses two ways to accomplish reduction or elimination of such bearing error by compensation for the amplitude modulation of the source signal.